The testing of digital circuitry presents a number of challenges not found in the area of analog circuit testing. In analog circuit testing a circuit can be characterized by various voltage levels or current levels at certain test points or by the frequencies of certain signals at those test points. In addition, most analog circuits have known or measurable transfer functions which can be analyzed with the help of instruments such as network analyzers. Although also used in conjunction with digital circuits, the foregoing types of measurements are not as helpful in characterizing, analyzing, or diagnosing digital circuitry. The signals of interest in an operating digital system are usually a series of pulses or binary digits which have a certain time relationship to a clock in the system. The informational content of these digital signals or data streams is usually of much more interest than the particular voltage or current levels or the repetition rate of the signal.
Voltage or current level measurement is used on connection with digital circuits to determine that the circuit is at least operating is some manner, and for this purpose devices such as logic probes that give a visual indication of the presence of pulses are used by logic designers and technicians. Depending upon the repetition rate of the pulses, voltmeters and oscilloscopes may also be used for this purpose; and, indeed, and oscilloscope may also be used to display a series of pulses. Level indications by voltmeters and logic probes are not very helpful, however, when it comes to determining the correctness of a digital signal comprising a stream of bits. Oscilloscopes are somewhat more helpful because they can display a series of pulses and can display the series of pulses in relationship with other pulses in the system. Oscilloscopes, however, are also limited in the number of pulses that can be conveniently displayed on the screen. Even with a high display capacity, however, oscilloscopes also have the shortcoming in that the user is required to compare the long series of pulses displayed, bit by bit, with the series that should be displayed for a correctly operating circuit. With the long data streams used in modern digital instruments such as calculators and computers this comparison can be very tedious process.
Other kinds of test instruments are also used to detect some minimal level of operation by the circuit. In this category are instruments such as frequency or event counters and transition counters. The frequency counter tells the repetition rate of the signal from the circuit being tested and is useful for verifying proper operation of a system clock. However, such a counter does not help in determining the correctness of a digital signal. Event or ones counters and transition counters are somewhat more helpful since the number of ones or transitions in the data stream from a properly operating circuit can be easily predicted and measured. However, such a test is by no means foolproof since a number of errors can arise that change desired sequence of bits in a digital signal but do not change the number of ones or transitions in it.
Because of the limitations enumerated above, it is very difficult for a test technician to locate a faulty circuit in a large or complex digital system with currently available test equipment. Often the test technician is faced with having to replace one circuit at a time until the system works properly. This method can be very time-consuming and requires a large inventory of spare circuits which the test technician must have available. Even after the faulty circuit has been replaced it is often difficult to isolate the problem and repair the circuit. The result is that expensive circuits may have to be discarded.